A bird watcher whose childhood interest in butterflies was recently revived, Bruce deGraaf has noticed growing numbers of "painted ladies’’ laying eggs on the milkweed growing in his backyard.

But nothing prepared him for the "unbelievable explosion of giant swallowtail butterflies’’ he’s seeing this summer.

"You occasionally see them in warmer years. But I’ve had them in my backyard,’’ said the retired engineer from Shrewsbury. "They’re staggeringly big. Unbelievably beautiful.’’

Across the state, hobbyists and dedicated lepidopterists like Steve Moore heard reports two years ago from the caretaker at Bartholomew Cobble, a wildlife refuge in Sheffield in southwestern Massachusetts, of dozens and then hundreds of the flamboyantly colored butterfly with the bright yellow band.

"Sometimes it takes a few years to know what you’re seeing,’’ said the semi-retired corporate attorney from Northborough. "There’s been a gradual increase over the last three or four years in what we call the ‘Southern species,’ like the zabulon skipper. Now they’re all over the place.’’

While butterfly watchers like deGraaf and Moore are thrilled by the arrival of gorgeous new species, both expressed "real concerns’’ about apparent climate changes that are driving familiar butterflies out of state even as new varieties arrive.

Their observations are confirmed by a Harvard University study that found the warming climate in Massachusetts, over the last 19 years, has been changing – sometimes dramatically – the number of different species of butterflies across the state.

Some familiar varieties have declined by as much as 90 percent while Southern species, including the frosted elfin, are arriving from Texas.

One of the report’s three authors, Elizabeth E. Crone, a senior ecologist at the Harvard Forest in Petersham, said 17 of 21 butterfly species familiar to Massachusetts are "declining" and generally moving northward and "14 new species are arriving from the South.’’

"We know there have been pretty striking changes over the last 20 years. We don’t know if things are changing faster than in the past,’’ she said.

In addition to Crone, the report was written by Greg A. Breed, a post-doctoral fellow at the Harvest Forest, and Sharon Stichter, of the Massachusetts Butterfly Club.

While focusing on butterflies, the report, "Climate-driven Changes in Northeastern U.S. Butterfly Communities,’’ published in Nature Climate Change, raises complex questions and concerns about what appears to be a dramatically changing climate and its implications for other creatures and their habitats.

Crone said the report was able to make crucial use of nearly 20,000 species counts over the last 19 years by members of the Massachusetts Butterfly Club. While many researchers would look skeptically on data gathered by volunteer naturalists, she said new statistical methods pioneered by Australian statisticians established a new method to reliably evaluate data based on the number of carefully observed species.

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While saying she was "worried’’ about Massachusetts’ rapidly changing climate, Crone said the report offered "good and bad’’ news about the shifting butterfly populations and what that might bode for the future.

"Butterflies are able to respond quickly to temperature change. I suspect we’re seeing a broad scale reorganization of butterfly communities,’’ she said. "Most scientists have known the climate is warming. This isn’t a surprise to scientists who’ve been studying this. … However, whenever things are rearranged, unexpected things might happen. Things could change in unexpected ways.’’

While climate routinely fluctuates around the world, average temperatures in Massachusetts have increased by 3.6 degrees Fahrenheit since 1900, though changing butterfly populations appear driven by a variety of factors.

While warmer local weather is driving butterfly populations, Moore, president of the MBC, said "it’s hard to say’’ whether it’s caused by increased carbon dioxide emissions or "natural cycles.’’ But deGraaf asked if a 3.6-degree change could drive butterfly populations, "what might be next?’’

George Leslie, founding owner of the Butterfly Place in Westford, is skeptical.

A butterfly collector for nearly 50 years, he described the shifting population as "just an aberration.’’

"Giant swallowtails have been showing up in Massachusetts for years. Monarchs are migratory. Some years you don’t see them. Other times they’re plentiful. We’ve had a mild winter. There’s drought in the South and Midwest. Butterflies forage for food. Females must find a host plant to lay their eggs on. I don’t think it’s a permanent thing. I’d be very skeptical (giant swallowtails) will be around in three to five years,’’ he said.

However, staffers at several MetroWest botanical gardens have observed other climate-driven changes affecting the interwoven web of plants and wildlife.

Bonnie Drexler, education director at Garden in the Woods, said staffers have seen fewer great spangled fritillaries this year but equal numbers of monarchs.

She has also observed that the peak blooming season for trillium and several woodland wildflowers has accelerated. When she began working at the Framingham garden 18 years ago, the peak was from mid-May to June. Now the peak blooming season is the end of April to the first part of May.

"Things are definitely changing. And things are blooming earlier. Birds feed on insects that feed on plants. Everything is all tied together,’’ said Drexler.

As blooming cycles shift, she said, there’s "renewed interest’’ in the field of phenology, the study of plant and animal life cycles, because minor changes often trigger unanticipated changes down the road.

"It’s much earlier this year. Usually things even out but this year it’s not evening out. This year we’re consistently ahead,’’ she said.

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Drexler and Viera stressed the mild winter and wet spring could have triggered an earlier blooming season, but both felt this year’s dramatic changes set in motion other changes in plants, birds and insect systems.

"When something impacts the ecosystem, it can have complicated effects one or two steps away. It’s not just what scientists tell us but what I see with my own eyes,’’ he said from the Arcadia Wildlife Sanctuary in Easthampton. "We should definitely be paying attention to these issues. If we avoid these projections, there can be serious ramifications down the line.’’